1. Field of the Invention
The present invention relates generally to bubble jet printing systems that use the volume change of bubbles produced by heating in order to spray small jets of conductive ink, and more particularly to an ink-jet head of such an ink-jet printer of the so-called "current-flow" type wherein a current is passed through a conductive ink to cause the ink to become vaporized and cause any trapped gases or bubbles to expand, forcing droplets of ink to spout from the ink-jet head.
2. Description of the Prior Art
With a growing demand for a quick, quiet and color-printout printer, interest has been shown towards ink-jet printers recently.
The ink-jet printers are generally classified into two groups, one being the continuous type and the other being the on-demand type. The on-demand type ink-jet printer is further divided, according to the driving system, into Kyser type, Stemme type, and Gould type, all types that are driven by a piezoelectric device, and a bubble jet type in which a conductive ink is ejected using volume change of a bubble produced on heating.
The bubble jet type ink-jet printer is further classified into two groups according to the heating system. The first group is called "thermal head type" in which a heater is used for heating the conductive ink, while the second group is called "current-flow type" using a current flowing through the conductive ink to cause the conductive ink to generate heat,
A typical example of ink-jet heads used in the conventional current-flow type ink-jet printers will be described below with reference to FIGS. 6 and 7 of the accompanying drawings.
FIG. 6 is a fragmentary cross-sectional view of the conventional ink-jet head, and FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 6. The ink-jet head includes a base plate 1 on which a heat-insulating layer 2 is laminated. A nozzle plate 4 having a plurality of discharge holes 3 (only one being shown) is disposed in parallel spaced relation to the base plate 1. A plurality of pairs of opposed electrodes 5 (only one electrode pair being shown) arranged in a predetermined pattern or matrix are disposed on an upper surface of the heat-insulating layer 2, with each of the individual pairs of electrodes 5 being located at a position corresponding to the position of one of the discharge holes 3. Each of the electrode pairs 5 is composed of a signal electrode 5A and a common electrode 5b confronting one another with a predetermined space or gap therebetween. A partition member 6 made of an electrically insulating material is disposed between the heat-insulating layer 2 and the nozzle plate 4 so as to isolate the individual discharge holes 3 from one another against interference. The heat-insulating layer 2, partition member 6 and nozzle plate 4 jointly define therebetween a plurality of pressure chambers 7 (only one being shown) each of which communicates with an ink passage 8, so that a conductive ink is introduced into the pressure chamber 7 through the ink passage 8. In FIG. 6, reference numeral 10 is a pulse voltage generator by means of which a voltage from a DC power supply 11 is selectively applied to the opposed electrodes 5.
Reference character a designates electric lines of force, also known as electric flux lines, passing through a portion of the conductive ink contained between the signal electrode 5A and the common electrode 5B. The signal electrodes 5A and the common electrode 5B define therebetween a current flow passage A which is dimensioned by the distance D (FIG. 7) between the electrodes 5A, 5B, the width W of the electrodes 5A, 5B and the height H (FIG. 6) of the electrodes 5A, 5B.
With this arrangement, when a current flows along the electric lines of force a, the conductive ink contained in the current flow channel A is caused to evolve heat and become vaporized, thereby producing bubbles. The bubbles thus formed raises the pressure in the pressure chamber 7, forcing a droplet 9 of ink from the discharge hole 3 of the ink-jet head.
Operation of the conventional ink-jet head of the foregoing construction will be described below in greater detail.
When a pulse voltage is applied from the pulse voltage generator 10 to the signal electrode 5A and the common electrode 5B, a current is passed through the current flow passage A along electric lines a of force created between the electrodes 5A and 5B. The current causes that portion of the conductive ink contained in the current flow passage A to evolve heat and then become vaporized, thereby producing bubbles (not shown). Due to the bubbles thus produced, the pressure in the pressure chamber 7 rises suddenly so that a droplet 9 of ink is forced from the discharge hole 3 onto the surface of a recording paper (not shown) placed above the ink-jet head, thus forming a dot of a character to be printed on the recording paper. After ejection of the conductive ink, an adequate amount of conductive ink is replenished from the ink passage 8 to the pressure chamber 7 so that the pressure chamber 7 is always filled with the conductive ink.
The conventional ink-jet head, however, has drawbacks as described below. Due to the shape and configuration of the current flow passage A, the conductive ink contained in the current flow passage A generates heat uniformly over the entire area thereof. This means that the whole of the conductive ink contained in the current flow channel A are participated in the generation of heat and must evolve heat above a boiling point in order to produce bubbles. Thus, a large amount of power is dissipated as heat and hence the energy efficiency is low. In addition, due to the uneven shape and configuration of the electrodes 5, due to deterioration of the electrodes 5, or due to the uneven shape and configuration of the pressure chambers 7, bubbles produced on boiling of the conductive ink tend to distribute unevenly over the entire area of each of the current flow passage A. With this uneven distribution of bubbles, droplets of ink are liable to be ejected in different directions and with different sizes. In order to produce uniformly distributed bubbles, the distance D (FIG. 7) between the signal electrode 5A and the common electrode 5B and the width W of the electrodes 5A, 5B must be smaller than 50 .mu.m, or end surfaces of the respective electrodes 5A, 5B must be finished precisely. In either case, an additional production cost is needed, resulting in an expensive ink-jet heat.